A single word or very short line left alone at the end of a paragraph, visually disconnected from the rest of the text block.

Example Usage

Rewrite the headline to avoid the widow: ‘your’ shouldn’t sit on its own line.
Fig 3.8 Widow

Widows make copy feel unfinished. The fix is almost never CSS. Rewrite the sentence until the line breaks fall where you want them. text-wrap: balance helps at the margins.